Gamma ray imaging systems have many applications in the field of non-destructive internal viewing of objects. In particular, medical imaging systems, such as computer aided tomography (CAT), positron emission tomography (PET), and single photon emission computer tomography (SPECT), often use an array of scintillation detectors for sensing radiation. Each time it absorbs gamma radiation, a scintillating detector, such as a crystal of BGO (Bi.sub.4 GE.sub.3 O.sub.12), for example, emits light with intensity proportional to the energy of the absorbed gamma ray. The scintillating crystals of such detectors generally have diffuse reflecting coatings on all sides except for one bare face of each crystal that is in optical contact with a photomultiplier tube (PMT). Typical photon counting efficiency obtained for such systems using BGO crystals is about 10%, comprising the product of PMT photon collection efficiency of about 50% and PMT photo detection efficiency (QE) of about 20%.
Until recently, fiber optic collection of light from scintillating crystals used for gamma ray and X ray detection was considered too inefficient compared with direct collection by photomultiplier tubes (PMTs). U.S. Pat. No. 5,138,165 issued to Petroff describes a three-dimensional positron emission tomography system that uses an array of scintillating crystals in combination with solid state visible light photon counters (VLPCs). In this system, each scintillating crystal is coated with a layer of a diffuse reflector, such as MgO, except for a 1.times.1 mm.sup.2 opening facing a VLPC. Although this system replaces PMTs with solid state VLPCs, one VLPC is required for each of the multiplicity of scintillating crystals in the detector array. Because fiber readout of large arrays of small scintillating crystals has been considered too costly due to the large number of VLPC channels required, there remains a need for medical imaging systems having large detector arrays that provide greater scanning volume, improved resolution, and lower patient radiation doses.